The present invention relates generally to fluid control and, more particularly, to a throttling device or flow control element adapted for reducing the velocity of fluid flowing therethrough and which may be economically mass produced with a plurality of multi-axis internal flow passages. The flow control element is formed as a high strength unitary structure by direct metal laser sintering of successive layers of powdered material such that the multi-axis flow passages can be formed with highly complex geometries.
An exemplary application into which the flow control element may be integrated is similar to that which is disclosed in commonly owned U.S. Pat. No. 5,687,763. The '763 patent discloses a fluid control device including a valve structure having a flow control element disposed therewithin. An axially movable valve plug is slideably mounted within the interior of the valve housing. The flow control element disclosed in the '763 patent is comprised of a stack of annular disks that collectively define a series of substantially radially directed passageways extending between the inner and outer radial surfaces or edges of the disks. Each of the substantially radially directed passageways has a plurality of right angle turns formed therewithin in order to reduce the velocity of fluid that is flowing through the flow control element.
As disclosed in the '763 patent, the individual disks may be formed with partial or complete passages. Abutting or adjacently disposed disks having partial passageways may cooperate with one another to fully define the radial passageways of the flow control element. Likewise, abutting disks having complete passages formed in a radial direction may cooperate with adjacent disks stacked above or below in order to fully define the radial passageways of the flow control element. In this regard, the abutting disks act to close off communication of fluid within the passageways of the subject disk by utilizing planar faces of the adjacent disks.
Ideally, the radial passageways are tortuous in nature in that the passageways contain a substantial number of turns in order to reduce the velocity of fluid flowing through the flow control element. As disclosed in the '763 patent, the flow control element may be configured to reduce the velocity of fluid flowing in an inward direction. Alternatively, the flow control element may be configured for reducing the velocity of fluid flowing in an outward direction.
In fabricating such flow control elements, one type of practice to date has been to pre-machine individual hardened disks and then to align and assemble the hardened disks in a stack in order to form the flow control element having the plurality of passageways. The securement of the stack of hardened disks has been either by the use of tension rods, as is shown in the '763 patent, or by the use of brazing materials wherein the bearing surfaces such as the outer peripheral edges of the adjacent disks are brazed together in order to assemble the stack of disks. In the case of tungsten carbide disks or cylinders, the assembled stack may appear as a unitary or one-piece element although the element is actually comprised of multiple disks or cylinders that are brazed or otherwise attached to one another.
One of the advantages of utilizing tungsten carbide to form the flow control element is its superior erosion resistance. Particularly in severe service applications wherein entrained sand may be captured in fluid which flows through a valve assembly, the flow control element must have a very high resistance to erosion from the entrained sand. In this regard, tungsten carbide is the hardest known element with a compressive strength that is greater than that of any other metal or alloy making tungsten carbide well suited for abrasion resistant applications.
However, a significant drawback to the use of tungsten carbide and other materials in a flow control element is the difficulty associated with machining the material due to its extreme hardness. For example, certain alloys such as alloys of stainless steel and tungsten carbide can only be machined by using diamond-grit grinding devices. Furthermore, a substantial cost in using disks of tungsten carbide is the requirement for grinding them flat prior to assembly into the disk stack. The disks must be ground flat in order to ensure proper registry and alignment of the disks in the stack formation in order to prevent a “tacoing” effect wherein disks in the stack may warp. The tacoing effect can also occur when using stainless steel sheet metal with electron discharge machining (EDM) or punching disks.
Warps between adjacent disks may result in gaps between the disks which can allow fluid to escape. The escaping fluid can result in an overall reduction in the energy dissipating capability of the disk stack. In addition, as high pressure fluid flows through gaps between the warped disks, erosion of the adjacent disks can occur which, over time, can further reduce the energy dissipating capability of the disk stack and can compromise the overall performance of the flow control element and, ultimately, the overall control of the valve which could have an effect on the entire system.
In an attempt to overcome the above-noted drawbacks associated with assembling pre-machined disks of hardened material, alternative practices for manufacturing flow control elements have been developed in the prior art, including the use of green state technology. More specifically, in a green state manufacturing process, the flow control element is formed by a series of pre-formed individual annular green state disks fabricated from metallic or ceramic powder mixture (such as stainless steel or tungsten carbide powder mixture) in an unsintered green state. Binder material is added to the powder mixture to aid in permanently bonding particles of the powder mixture during sintering of the green state disks.
The individual green state disks have partial or complete disk passages formed therein. The series of green state disks are assembled in a stacked formation so that the partial or complete disk passages of adjacently-disposed green state disks form the substantially radial device passageways. The partial passages are formed in the disks prior to the assembly of the individual green state disks. The individual green state disks are assembled in the stacked formation prior to the hardening of the disks by heat. The assembled stack of green state disks are sintered or heated as a unit in order to unitize the individual green state disks together into the flow control element.
Although the above-described green state manufacturing process overcomes some of the difficulties associated with the machining and assembly of hardened material, the green state process includes inherent limitations related to the forming of the internal flow passages. More specifically, in some flow control elements, it is desirable to provide highly complex flow paths in the internal flow passages such that the internal flow passages have a plurality of multi-axis right angle turns in order to optimize fluid control characteristics for a given application.
Unfortunately, forming multi-axis flow paths using conventional disk stack technology or using green state technology is either not possible or is extremely expensive due to the required tooling investment and substantial manufacturing and assembly time. Furthermore, fabrication practices using known disk stack technology or using green state technology results in limitations in the overall strength such as in the ability of the disk stack to resist hoop stresses and the limitations of the braze strength. Even further, material waste is relatively high in conventional disk stack technology as a result of the discarding of support material used in the manufacturing process. Conventional brazing operations used in assembling disk stacks also present certain deficiencies related to the clogging or partial blocking of the passageways in the disk and the limitations of the braze strength. As mentioned above, fluid leakage may also occur using conventional disk stack technology as a result of the “tacoing” effect which may arise as a result of wavy or non-planar sheet stock, excessive fluid pressure within the flow control element, or as a result of low quality brazing. The buildup of differential stresses may also occur as a result of heat treating the disk stack subsequent to the stacking of the individual pieces or disks therein.
As can be seen, there exists a need in the art for an improved technique for manufacturing a flow control element of the type that may be used in valve assemblies or fluid control devices. More particularly, there exists a need in the art for a flow control element that can be fabricated with a plurality of complex multi-axis or multi-directional internal flow passages in a unitary structure. Furthermore, there exists a need in the art for a flow control element that can be economically mass-produced with a high degree of accuracy and repeatability and with improved strength properties.